FIG. 10 is a schematic sectional view of a conventional elastic wave device. In FIG. 10, conventional elastic wave device 1 includes piezoelectric substrate 2 made of lithium niobate, IDT (Interdigital Transducer) electrode 3 disposed on piezoelectric substrate 2, first dielectric layer 4 made of silicon oxide and disposed on piezoelectric substrate 2 such that it covers IDT electrode 3, and second dielectric layer 5 made of a medium that propagates transverse waves faster than that on first dielectric layer 4. Second dielectric layer 5 is disposed on first dielectric layer 4.
A known prior art related to the present invention is, for example,
Japanese Patent Unexamined Publication No. 2007-267366.
The above conventional elastic wave device 1 suppresses an electromechanical coupling coefficient of Stoneley wave, which is an undesired wave, by specifying a film thickness of first dielectric layer 4, a film thickness of IDT electrode 3, and a cut angle of the substrate. However, an electromechanical coupling coefficient of SH wave, which is a major wave, is only 0.15 at maximum. Therefore, if conventional elastic wave device 1 is applied to a duplexer having a wide transmission gap, such as Band I of UMTS, it is difficult to gain good filter characteristics.